I think I'd have to say NextJS with SQLite. Every DB table would just be 'id' and 'data', where 'data' is JSON-encoded and the schema is only enforced in code. This lets you write a 30-line ORM. Foreign-key relationships are just modelled as lists. If speed is the major concern, I find this the fastest way to build. You can bring structure to the DB as things are nailed down.